Flagging won't do anything. Moderators can't unaccept an answer. In very extreme cases, moderators can delete accepted answers but I can't imagine a time where this would be necessary here. There are many other options.
Users should be smart enough to see an answer that is accepted but low-scoring as a poorer answer than one that is high scoring but not accepted.
You can leave a comment for the OP to let them know that the accepted answer is incorrect but that will only do anything if they are still active on the site and agree with your interpretation.
You can leave a comment on the answer to explain that it is wrong and that other answers should be used instead. You are also welcome to post the answer in the ELL chat room for discussion and, if others agree with you, they may also vote down the answer.
Sometimes I see cases where someone vehemently disagrees on the validity of an answer but that does not always make the answer wrong. English is a funny language and there are occasionally multiple correct answers depending on how you look at things.
As a note, feel free to review the related question on MSE.
It notes a couple of other options that I'm not sure are necessarily appropriate but, for example, you can edit an answer to fix it rather than writing your own. If you truly believe that the accepted answer is doing harm to users, that may be the better option, though be aware that if the person who posted the answer is active, they may not like your edit and may roll it back.... though, if they are active, one would hope that they would accept the possibility that they are incorrect and you could have a dialogue regarding fixing the answer.